Select contributor terms from a list of the available jurisdictions

Select contributor terms from a list of the available jurisdictions

Rather than probing the filesystem with an interpolated path and falling
back when it doesn't exist, check the country code against the set of
files in config/legales, so that only known-good values are ever joined
onto a path. This also covers codes returned by OSM.ip_to_country, which
the controller's parameter validation doesn't apply to.